Generally, a BWR is provided with what they call a jet pump system to achieve a high power density. The jet pump system is formed by combining a recirculation pump installed outside a reactor pressure vessel and jet pumps installed inside the reactor pressure vessel. A jet pump installed in a BWR employing a jet pump system will be described with reference to FIGS. 28 and 29.
FIG. 28 is a schematic longitudinal sectional view of a BWR. A coolant 2 is contained in and a core 3 is installed in a reactor pressure vessel 1. The core 3 includes fuel assemblies and control rods, which are not shown. The core 3 is installed in a core shroud 10.
The coolant 2 is heated by nuclear reaction heat generated by the core 3 as the coolant 2 flows upward through the core 3. Thus the coolant 2 is converted into a two-phase flow of water and steam. The two-phase flow of the coolant 2 flows into a steam separator 4 disposed above the core 3. The steam separator 4 separates steam from the two-phase coolant 2. Then, the separated steam flows into a steam dryer 5 disposed above the steam separator 4 and is dried by the steam dryer 5 to produce dry steam. The dry steam is supplied through a main steam line connected to the reactor pressure vessel 1 to a steam turbine, not shown, for power generation. Water flows down through a downcomer 7 between the core 3 and the reactor pressure vessel 1 into a space below the core 3.
Control rod guide pipes 8 are installed below the core 3. The control rod guide pipes 8 guides the control rods when the control rods are inserted into and withdrawn from the core 3. A control rod drive mechanism 9 is installed below the control rod guide pipes 8. The control rod drive mechanism 9 drives the control rods to insert the control rods into and to withdraw the control rods from the core 3.
Plural jet pumps 11 are arranged at equal angular intervals in the downcomer 7.
A recirculation pump, not shown, is installed outside the reactor pressure vessel 1. The recirculation pump, the jet pumps 11 and recirculation lines connecting the jet pumps 11 to the recirculation pump constitute a recirculation system. The recirculation pump supplies driving water to the jet pumps 11 to cause the forced circulation of the coolant 2 in the core by the agency of the jet pumps 11.
FIG. 29 shows an important part of FIG. 28 in an enlarged view. Referring to FIG. 29, the jet pump 11 has a riser 12. The riser 12 is fixed to the reactor pressure vessel 11. The coolant 2 supplied through a recirculation inlet nozzle 13 included in the recirculation pump is introduced into the reactor through the riser 12.
A pair of elbows 15A and 15B is connected to an upper part of the riser 12 by a transition piece 14. The elbows 15A and 15B are connected through mixing nozzles 16A and 16B to inlet throats 17A and 17B, respectively. Diffusers 18A and 18B are connected to the inlet throats 17A and 17B, respectively.
When the coolant 2 is jetted through the mixing nozzles 16A and 16B, jets of the coolant 2 entrain water around the jet pump 11. The jetted coolant 2 and the water entrained by the jetted coolant 2 are mixed in the inlet throats 17A and 17B. Then, the diffusers 18a and 18B recover a hydrostatic head.
The flow of the coolant pumped into the reactor pressure vessel 1 by the recirculation pump generates fluid vibrations. To cope with the fluid vibrations, the lower end of the riser 12 is welded to the recirculation inlet nozzle 13, and the upper end of the riser 12 is fixedly connected to the reactor pressure vessel 1 by a riser brace 20.
The upper ends of the inlet throats 17A and 17B are mechanically connected to the transition piece 14 by the mixing nozzles 16A and 16B and the bends, respectively. The lower ends of the inlet throats 17A and 17B are inserted into the upper parts of the diffusers 18A and 18B, respectively. The riser 12 and the inlet throats 17A and 17B thus held so as to be capable of satisfactorily withstand the fluid vibrations.
Upper parts of the mixing nozzles 16A and 16B will be described. A pair of ears 21 is formed on the opposite sides of the transition piece 14. The ears 21 extend upward so as to define a groove 22 between upper parts of the ears 21. A pair of jet pump beams 23 having a rectangular cross section enlarging in a longitudinal direction toward a middle part is fixedly placed in the groove 22 with the opposite ends thereof fitted in the groove 22. The jet pump beams 23 are provided in their central parts with vertical threaded holes, not shown, respectively. Head bolts are screwed into the threaded holes, respectively. Each of the head bolts 28 has a hexagon head and a semispherical tip.
Horizontal seats, not shown, are formed in the upper ends of the elbows 15A and 16B, respectively. Counterbores, not shown, are formed in the seats. The semispherical tips of the head bolts 28 are fitted through spherical washers in the counterbores, respectively.
Since the inlet throats 17A and 17B are not fixed to the reactor pressure vessel 1, the pressure of the driving water supplied through the riser 12 works on the upper ends of the inlet throats 17A and 17B, and the elbows 15A and 15B. Reaction to the ejection of the driving water through nozzles, not shown, connected to the other ends of the elbows 15A and 15B into the diffusers 18A and 18B acts upward on the elbows 15a and 15B. The head bolts 28 are screwed into the threaded holes of the jet pump beams 23 to bear this force.
Since the ears 21 are held fixedly in place, the jet pump beams 23 move upward and the opposite ends thereof are pressed against upper walls defining the groove 22 as the head bolts 28 are screwed into the threaded holes to bear the upward force.
Downward force is exerted through the head bolts 28 on the upper ends of the elbows 15A and 15B. The magnitude of the downward force is dependent on the upward force, namely, the reaction to the ejection of the driving water through the nozzles. Keepers, not shown, are put on the hexagon heads of the head bolts 28. Each of the keepers is welded to a support plate, not shown, by spot welding. The support plate is quadrilateral and is fastened to the upper surface of the jet pump beam 23 with two bolts.
The inlet throats 17A and 17B are attached to a riser bracket 25 fixed to the riser 12. The diffusers 18A and 18B are fixed to a baffle plate 26 welded to the reactor pressure vessel 1.
The jet pump 11, as compared with other devices, is used under a severe condition. Therefore, as big load acts on the component members of the jet pump 11. A very high stress is induced in particular in the riser brace 20 holding the middle part of the riser 12.
The riser brace 20 suppresses fluid vibrations generated in the riser 12 while the BWR is in operation, and absorbs a difference in thermal expansion between the reactor pressure vessel 1 made of a carbon steel and a riser 12 made of an austenitic stainless steel. Therefore, the riser brace 20 absorbing the difference in thermal expansion deforms while the BWR is in operation.
Measurement of the flow of the driving water through the jet pump during a normal operation is important for controlling a nuclear power plant. Metering pipes 19 are attached to upper and lower parts, respectively, of the diffusers 18A and 18B to measure a static pressure difference between the upper and the lower part of the diffuser 18 during operation. The measured static pressure difference is compared with calibrated values determined before the plant starts operating to calculate a driving water flow in the jet pump.
The metering pipes 19 are fitted in static pressure measuring holes formed in upper and lower parts of the diffuser 18, are welded to the upper and the lower part of the diffuser, and are welded to connecting members 24 fixed to the diffuser 18. As shown in FIGS. 30(a) and 30(b). The metering pipes 19 are arranged in a complicated arrangement near lower parts of the jet pumps 11. The metering pipes 19 are connected to jet pump measuring nozzles 27 connected to an external line. The jet pump measuring nozzles 27 are disposed symmetrically on the reactor pressure vessel 1.
The jet pumps 11 thus constructed are driven by the coolant supplied by the recirculation pump and are used under a severe condition as compared with other component devices. Therefore, high force acts on the component members. The metering pipes 19, in particular, are influenced directly or through the connecting members 24 by the fluid vibrations, and hence high stress is induced in the metering pipes 19. Thus, the breakage of the metering pipes 19 is expected at a high probability. The breakage of the metering pipes 19 causes problems in the output control of the BWR, and the broken metering pipes 19 need repairing.
As obvious from FIG. 30(b), the metering pipes 19 are arranged in an annular space 29 between the reactor pressure vessel 1 and the shroud 10, the risers 11 and the inlet throats 17 are disposed above the metering pipes 19, and the component members of the jet pumps including the riser braces 20, the mixing nozzles 16 and the elbows 15 are disposed in the annular space 29.
A technique for prevent the propagation of pressure pulsation in a nuclear reactor is proposed in, for example, Patent document 1. A method of detecting cracks in the jet pump beams of a nuclear reactor is proposed in, for example, Patent document 2. Method of changing the parts of a jet pump is proposed in, for example, Patent document 3.
Patent document 1: JP H10-239479 A
Patent document 2: JP 2004-151097 A
Patent document 3: JP H8-201566